"Ellie killing Abby would've been like Joel dying all over again".... ...what? Joel had every reason to save Ellie from Abby's murderous father at the end of Tlou1, that death/choice was entirely justified and Abby is just a shitty person for not even being able to see why a father would want to save his daughter from certain death regardless of other factors. Ellie killing Abby would put an end to this threat who clearly doesn't care about anything other than her own feelings. Heck, had Joel been there he would've encouraged Ellie to finish the job since Abby was clearly too unhinged and dangerous to be left around.